This is what I would probably do for maximum flexibility & robustness:

*** tblComponents
(think of these as the RawItems or Parts that comprise your
Merchandise/Products.)
   -ComponentID (Unique, maybe PK)
   -ComponentDescription
   -Supplier
   -?ComponentCost
Inventory records (if any) would use this as their starting point.

*** tblProducts
(This reflects what is available for sale. They are comprised of one or more
components: single vs. case)
   -ProductID (Unique, maybe PK)
   -ProductDescription
   -?ProductCost
Sales records would use this as their starting point

*** tblProductComponents
(A Join table. Translates Products into Components & vice versa. What
Components go into a Product? A "single" product with a single component
will have one record here, a "case" with multiple components will have
multiple records)
   -ProductComponentID (AutonumberPK)
   -ProductID (many-to-one: tblProducts)
   -ComponentID (many-to-one: tblComponents)
   -ComponentQuantity (default is 1, but easily allows multiple components
in a single product: "two-for-one" specials)
The combination of + ProductID + ComponentID should be a unique index.
***

HTH,